Artificial transmission line formed by coiling plural line foils and shorted screening foil

ABSTRACT

A WOUND UP LOW INDUCTANCE ARTIFICIAL LINE IS FORMED BY DEPOSITING RESISTIVE FILMS ON DIELECTRIC MATERIAL WHICH IS THEN WOUND UP, LIKE A ROLL-TYPE CAPACITOR, WITH A SCREENING CONDUCTOR BETWEEN EACH ADJACENT TURN OF THE ROLL. EACH TURN OF THE SCREENING CONDUCTOR IS SHORT-CIRCUITED TO ELIMINATE ITS INDUCTANCE. THE SCREENING CONDUCTOR MAY BE ANOTHER RESISTIVE FILM ON A DIELECTRIC WEB.

Jan. 12,1971 RALPH ET AL 3,555,461

Filed Sept. 29,, 1967 ARTIFICAL TRANSMISSION LINE FORMED BY. COILING PLURAL LINE FOILS AND SHORTED SCREENING FOIL 2 Sheets-Sheet 1 /4 arvz\\ J f-W-{ [RALPH .JIL PARMEL n aoswcu 8, FR. Hu/vrurr Inventors Attorney Jan. 12, 1971 F. RALPH ET AL 3,555,461

ARTIFICAL TRANSMISSION LINE FORMED BY COILING PLURAL LINE FOILS AND SHORTED SCREENING FOIL Filed Sept. 29, 1967 2 Sheets-Sheet 2 NW3 /4 R 2 ,/4

United States atent O U.S. Cl. 33331 2 Claims ABSTRACT OF THE DISCLOSURE A wound up low inductance artificial line is formed by depositing resistive films on dielectric material which is then wound up, like a roll-type capacitor, with a screening conductor between each adjacent turn of the roll. Each turn of the screening conductor is short-circuited to eliminate its inductance. The screening conductor may be another resistive film on a dielectric web.

The invention relates to artificial transmission lines of low effective inductance, that is lines which can be considered, in the frequency range for which they are designed, to have only series resistance and shunt capacitance.

According to the present invention there is provided an artificial transmission line whose series resistance and shunt susceptance in the frequency band for which the line is designed are large compared to its series reactance and shunt conductance, respectively, the artificial line including, superimposed on one another and Wound up into a roll, one or a pair of line conductors provided by a film or films of resistive material deposited on a length or lengths of dielectric material and a length of screening conductor arranged to screen each turn of the line conductor or conductors from the adjacent turn, each turn of the screening conductor being short circuited.

Embodiments of the invention will be described with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic perspective view of an artificial line according to the invention;

FIG. 2 is a developed plan view of a web of the assembly of FIG. 1;

FIG. 3 is a diagrammatic longitudinal view of an assembly of webs which are rolled up together to form an embodiment of the invention;

FIG. 4 is a cross-section in the plane IV-IV of FIG. 3;

FIG. 5 illustrates the equivalent circuit diagram of the embodiment of FIGS. 1 to 4;

FIG. 6 is a diagrammatic cross-sectional view, similar to that of FIG. 4, illustrating a modification of the embodiments of FIGS. 3 and 4; and

FIGS. 7 and 8, which are views corresponding respectively to those of FIGS. 4 and 3, illustrate another embodiment of the invention.

In the arrangement of FIGS. 1 to 4 four webs of dielectric material, 1 to 4, are each coated on their upper surfaces, as viewed in the drawings, with films 5, 6, 7 and 8, respectively, of resistive material. A plan view of the web 2 is shown in FIG. 2. It is of length l and the film 6, of width w, slightly less than the width of the web, extends from end to end. At each end terminal tapes 9, 10, respectively, are laid across the coating 6 so that when the web is rolled up into the roll of FIG. 1 these terminal tapes will project from one end face of the roll.

Patented Jan. 12, 1971 The web 3 is exactly similar to web 2 and is provided with terminals 11 and 12 to the ends of the film 7. The webs 1 and 4 carry film coatings 5 and 8, respectively, which screen adjacent turns of the rolled up assembly from one another. As shown in FIG. 4 they extend beyond the ends of the films 6 and 7 to either side, being taken right to one edge of their web but leaving a small uncoated margin at their other edge. Thus in the web assembly, uncoated edges of the dielectric web form the end face of the roll of FIG. 1 from which the terminal tapes 9 to 12 project. To prevent short circuits between these terminal tapes, they are provided with insulation adjacent the webs as indicated at 13 in FIG. 2. After the webs have been wound up into a roll, the end face to which the screening conductor films 7 and 8 are flush is sprayed or otherwise coated with low resistance material, indicated at 14 in FIG. 4, so that the whole length of each of the films 5 and 8 is shorted out by the coating 14 and, in particular, each turn of the winding of the screening conductors is short circuited. This ensures that, though the films 5 and 8 may be resistive films, their effective resistance throughout the length of the line is very small, while the short circuiting of each turn of the winding eliminates their self-inductance and also provides short circuited turns closely coupled with the line conductors 6 and 7 of the artificial line. If a thin dielectric be used for the web 2, the conductors 6 and 7 are closely coupled magnetically so that the self-inductance of each is substantially cancelled out by the mutual inductance between them, while the inductive effects of the winding are still further reduced by the short circuited turns of the screening conductors;

In the case of an unbalanced line, one of the webs 2 or 3 may be omitted. In this case, although there is now no magnetic cancellation between a pair of line conductors, as in the embodiment described above, the effect of the short circuited turns of the screening conductors will still keep the effective inductance of the unbalanced line low compared to its series resistance.

In FIG. 5 the distributed series resistance of the line conductors 6 and 7 of the embodiment of FIGS. 1 to 4 is represented by the resistors R, the capacitance to the dielectric 2 between the line conductors is shown by the capacitors C and the distributed capacitance of each line conductor to the adjacent screening conductor is indicated by the capacitors C Were it not for the short circuited screening conductors 5 and 8 there would be direct capacitance between adjacent turns of the wound up unit of FIG. 1, so that, in place of pairs of capacitors C in series with one another across the two line conductors there would be capacitance from each point of one of the line conductors to a point further along on the other line conductor, then there would effectively be diagonal cross capacitances in the equivalent circuit net- Work. Such a network could at best approxiate to a practical transmission line such as provided by a telephone subscriber cable. By the present invention, however, the characteristics of such a telephone cable can be simulated to a high degree of accuracy.

It is to be noted that when the web arrangement of FIG. 3 is wound into a roll the adjacent turns the screening conductors 5 and 8, separated by the dielectric web 4, come adjacent to one another and hence, for the turns intermediate the ends of the winding, one of these is redundant. One of the screening conductors 5 or 8 and its dielectric web 1 or 4, respectively, could therefore be omitted provided that an extra turn of the remainingscreening conductor is provided to complete the screening of the ends of the winding. If, say, the web 1 with its screening film 5 is omitted, and the assembly as shown in FIG. 3 is wound up with the right hand end innermost,

the screening of all except the innermost turn of the line conductor 6 will be effected by screening conductor 8. To complete the screening for this innermost turn the web 4 and its coating 8 should be continued to the right as viewed in the drawing, by a length equivalent to one further turn which then will overlay the innermost turn of film conductor 6. Although such an arrangement will decrease slightly the bulk of the wound up unit of artificial line, it is convenient, for measurement purposes, to make tests on the artificial line before it is wound up. If the web 1 be omitted, when the artificial line is laid out fiat the set of capacitances C associated with conductor 5 in FIG. 5 will be absent and hence the capacitance per unit length of the fiat artificial line will not be the same as that in its final rolled up form. Furthermore, it is mechanically simpler to wind together pairs of webs than to have to wind up an odd number of webs into a single roll. Thus in a practical application the advantages of retaining the theoretically redundant web 1 or 4 may be outweighed by practical considerations of manufacture.

Mention has been made above of the simulation of a length of telephone subscriber cable by an artificial line according to the invention. The design of an embodiment of FIGS. 1 to 4 to simulate a given length of a particular design of telephone cable will now be described.

The standard unloaded subscribers telephone cable can be represented as a transmission line having series resistance and shunt capacitance. The characteristic impedance Z of such a line is given by where R is the resistance per loop mile and C is the capacitance per mile. For one mile of a typical design of telephone subscriber cable having copper conductors of weight 4 lbs. per mile R =440 ohms and C the capacitance per mile, =0.069 ,uf.

If R be the sheet resistivity in ohms/ square of the conductive films 6 and 7 the resistance per unit length of the artificial line is 2R /w, for 6 and 7 are effectively in series when the artificial line is connected in circuit. If C be the direct capacitance per unit area between line conductors 6 and 7 separated by the dielectric web 2 (correpsonding to C in FIG. 5), and if the capacitance per unit area between films 5 and 6 and between films 7 and 8 (corresponding to C in FIG. 5) is also C the capacitance per unit length of the artificial line is 1.5 wC For the artificial line to simulate a length L of the telephone cable mentioned above, L being in miles, the total resistance and the total capacitance must be the same in each case. Thus the two following relationships are necessary:

from which If the characteristic impedance of the artificial line be expressed in terms of its loop resistance per unit length and capacitance per unit length it will be found that the expression for w given above is the necessary condition that this characteristic impedance be the same as that for the telephone cable.

A polycarbonate dielectric 6 microns thick yields a value of C :=4 10- uf./cm. while the sheet resistivity of evaporated aluminium film is typically 1.75 ohm/sq. Thus, for the artificial line of FIGS. 1 to 4 to simulate a 2 mile length of the above typical subscriber telephone cable having a loop resistance of 440 ohms/mile and a capacitance of 0.069 f./mile, using webs of metallised polycarbonate 6 microns thick, the required width and length of metallising is given by w=0.96 cm. and 1:240 cm.

For a width of metallising such as this, we prefer to have an uncoated margin of about 1 mm. at each edge of the metallising, so the width of the dielectric webs 1 to 4 can be made 1.2 cm. If desired, the dielectric webs may include additional unmetallised end portions. For the specific example given above, the rolled up artificial line unit, after being coated with a protective covering of resin, would have a diameter of about 1 cm., while the length of the roll of FIG. 1, corresponding to the width of the webs, would be about 1.4 cm.

To ensure a uniform capacitance per unit length in the artificial line it is necessary that when the webs are wound up into a roll the dielectric material of each web shall be in intimate contact with the conductive film on the adjoining web. For the avoidance of voids, such as air pockets, between adjacent webs, it may be arranged in embodiments of the invention that each constituent web of the wound up artificial line is provided with a conductive surface on each side so that, in the winding up process, pairs of conductors are brought into contact rather than a conductive surface and a dielectric surface. To this end the arrangement of FIGS. 3 and 4 described above may be modified as shown in FIG. 6, which is a view corresponding to that of FIG. 4. Here three dielectric Webs 15, 16 and 17 are used and each of them carries a film of conductive material on both sides. Thus web has a film 18, dimensioned similarly to film 5 of FIG. 4, on its upper surface as viewed in the drawing. On its lower surface the web 15 carries a conductive film 19 of width similar to the film 6 in FIGS. 3 and 4. Web 16 carries films and 21 on its opposite surfaces similarly arranged to films 6 and 7 of FIG. 4 while web 17 has a film 22 on its upper surface similar to the films 6 and 7 and a film 23 on its outer, lower surface similar to film 18 on web 15 or films 5 on web 1 of FIGS. 3 and 4. When the webs 15, 16 and 17 are wound up the films 19 and 20 will come together as also the films 21 and 22. These pairs of films then constitute the respective line condutors 6 and 7 of the preceding embodiment and are provided with terminal leads as in that embodiment. Between adjacent turns of the winding films 18 and 23 come together and constitute the shielding conductor between adjacent turns. As in the preceding embodiment terminal leads 10 and 12 for the ends of the line conductor windings are brought out from one end face of the rolled up assembly and the other face is coated with conductive material 14 so as to short out the whole length of the shielding conductor and in particular each turn thereof.

It has been mentioned above that it is preferable, mechanically, to wind together pairs of webs rather than an odd number of webs. An odd number of webs is necessarily involved in the embodiment of FIG. 6, but an equivalent alternative will now be described with refereence to FIGS. 7 and 8 wherein only two webs are involved.

In the embodiment of FIGS. 7 and 8 the screening conductor is a metal foil 24 which is coated on both sides with dielectric films 25 and 26, respectively. The films 25 and 26 may extend over the whole width and length of the foil. Superimposed on the dielectric films 25 and 26 are films of resistive material 27 and 28, respectively, of Width w to provide the required characteristic impedance of the artificial line and each of length l corresponding to the required length of line conductor. As will be explained later the foil 24 is somewhat longer than the length l and the films 27 and 28 extend from opposite ends of the foil as shown in FIG. 8. A second web 29 of dielectric material is coated on both faces with a conductive film 30 and 31, respectively, and both are shown in FIG. 8 extending the full length l of the web.

When the two webs 24 and 29 are wound up together into a roll, the conductive films 28 and 30 will be in contact with one another and, between adjacent turns of the roll films 27 and 31 will be brought in contact with one another. Thus the pair of conductive films 28 and 30 together form one line conductor equivalent to the line conductor 7 of FIGS. 3 and 4 while the films 27 and 31 together form the other line conductor equivalent to line conductor 6 of FIGS. 3 and 4. Assuming that the webs 24 and 29 are wound up together with their right hand ends as viewed in FIG. 8 at the inside of the winding, the commencement of films 27 and 28 adjacent terminal lead 10 will come into contact with the ends of film 31 and, at the outer end of the winding there will be a turn in which the outermost conductor is the foil 24. Looked at in another way, if the webs 24 and 29 were of the same length and the conductive films 27 and 31 extended the full length of the webs the first turn of the wound up assembly would carry on its inner surface an extra turn of the conductive film 27 while the outermost turn of the roll would carry an extra turn of the film conductor 28 which would not be cooperating with the film 30 and the dielectric of web 29 and which would not of course be shielded. Therefore at least this extra turn of the film conductor 28 must be removed from the web 24 and correspondingly, at the other end of the web 24 the excess length of film 27 should be removed, although this is not so important as at the outside of the assembly. It is of course immaterial whether or not the underlying dielectric film 25 or 26 is removed together with the conductive films 27, 28, respectively at the ends of the web 24.

Although, since the web 24 in the embodiment of FIGS. 7 and 8 is of metal foil, and therefore there is no need to apply a low resistive short circuit along the whole of its length as in the embodiments of FIGS. 3, 4, and 6, its self inductance must be kept low so that each turn of the foil must be short circuited. It will therefore sufiice, in this embodiment, if a short circuiting layer 14 be arranged to cover and make contact with all the turns of the web 24 along a radial strip on one face of the wound up assembly instead of covering the whole face. If the coating 14 be applied as one or more radial strips on one end face of the roll, one or more of the terminal tapes 9 to 12 may be brought out at this 'face.

The embodiments of FIG. 6 and FIGS. 7 and 8 have been described above in relation to a balanced line. If, in the arrangement of FIG. 6, an unbalanced line if desired, it is merely necessary to omit the web 16 from the wound up assembly while, in the embodiment of FIGS. 7 and 8 an unbalanced line may be obtained simply by omitting the web 29 with its coatings, the line conductor then being formed by the pair of resistive films 27 and 28 which contact one another throughout their length in the wound up assembly.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.

What we claim is:

1. An artificial transmission line comprising a plurality of films of resistive material each deposited on a respective sheet of dielectric material, said sheets being stacked in superimposed relation, terminal members extending from opposed edges of a first and a second film within said stack, said stack being wound into a spiral'roll with a further film of the stack forming a length of screening conductor interposed between each turn within said roll, said screening conductor at least coextensive with the surface of the dielectric sheet on which deposited, and a portion thereof extending further than said first and second films, and means for conductively joining each turn of the extending portion of the screening conductor.

2. An artificial transmission line as claimed in claim 1, wherein there are a plurality of terminal bearing films and a plurality of screening conductors and wherein the screening conductors comprise the top andbottom films of the superimposed stack.

References Cited UNITED STATES PATENTS OTHER REFERENCES Kimbark, E. W., Electrical Transmission of Power and Signals, J. Wiley & Sons, 1949, TK 3001 K55, pp. 426430.

Greer et al., IBM Technical Disclosure Bulletin, Vol. 3, #10, March 1961, pp. 35-36.

Barrett, R. M., Microwave Printed Circuits, A Historical Survey, IRE Trans. on Microwave Theory and Techniques, vol. MTT-3, March 1955, #2, pp. 1-8.

HERMAN KARL SAALBACH, Primary Examiner W. H. PUNTER, Assistant Examiner US. Cl. X.R. 

